1. Field of the Invention
This invention relates to a knock control system (KCS) for an engine which detects a knock occurrence in an engine and controls knock controlling factors (for example, ignition timing, air-fuel ratio and overfeed pressure).
2. Description of the Prior Art
The prior art has had problems leading into lowered precision of a knock judgement due to a deviation of the knock judgement level from the optimum value because of scatter in characteristics of engines, knock sensors and so on in their initial stage of usage, or changes of these characteristics with the passage of time. To solve this problem, a knock control system for an engine was proposed which can automatically correct the knock judgement level on the basis of the frequency distribution of the logarithmic conversion value of the knock sensor signal, because the frequency distribution of the logarithmic conversion value of the knock sensor signal becomes a normal distribution when no knock occurs, as disclosed in, for example, JP-A-267574/87.
Moreover, applicants of this invention filed another Japanese Application No. 147885/1988, in which the knock judgement level is updated many times so as to get fast convergence toward the optimum value by setting the knock judgement level in accordance with the standard deviation of the frequency distribution.
In the above system, however, in order to compute the standard deviation of the frequency distribution at a high speed enough for the actual control, it is necessary to burden a heavy load on computing means (for example, CPU) and storage means (for example, RAM and ROM). Therefore, there is the problem that it is difficult for this system to be applied to the concentrated control system (system for making ignition timing control, knock control, air-fuel control and so on by a single electronic control system) which has recently attracted public attention.